The use of a hand operated pointing device for use with a computer and its display has become almost universal. One form of the various types of pointing devices is the conventional (mechanical) louse, used in conjunction with a cooperating mouse pad. Mechanical mice typically include a rubber-surfaced steel ball that rolls over the mouse pad as the mouse is moved. Interior to the mouse are rollers, or wheels, that contact the ball at its equator and convert its rotation into electrical signals representing orthogonal components of mouse motion. These electrical signals are coupled to a computer, where software responds to the signals to change by a ΔX and a ΔY the displayed position of a pointer (cursor) in accordance with movement of the mouse.
In addition to mechanical types of pointing devices, such as a conventional mechanical mouse, optical pointing devices have also been developed. In one form of optical pointing device, rather than using a moving mechanical element like a ball, relative movement between an imaging surface, such as a finger or a desktop, and an image sensor within the optical pointing device, is optically sensed and converted into movement information.
Electronic image sensors, such as those typically employed in optical pointing devices, are predominantly of two types: charge coupled devices (CCDs) and complimentary metal oxide semiconductor—active pixel sensors (CMOS-APS). Both types of sensors typically contain an array of photodetectors (i.e., pixels), arranged in a pattern. Each individual photodetector operates to output a signal with a magnitude that is proportional to the intensity of light incident on the site of the photodetector. These output signals can then be subsequently processed and manipulated to generate an image that includes a plurality of individual picture elements (pixels), wherein each pixel in the image corresponds with one of the photodetectors (i.e., pixels) in the image sensor.
One form of optical pointing device includes an incoherent light source, such as a light emitting diode (LED), for illuminating an imaging or navigation surface to thereby generate reflected images which are sensed by the image sensor of the optical pointing device. Another form of optical pointing device includes a coherent light source, such as a laser, for illuminating an imaging surface to thereby generate reflective images to be sensed by the image sensor of the optical pointing device. Coherent light source based optical navigation with optical pointing devices can provide better imaging surface coverage and better tracking performance than provided with conventional incoherent light source optical pointing devices.
Coherent light sources, such as lasers, have significantly more stringent eye safety regulation than incoherent light sources, such as LEDs. For example, the International Electro-Technical Commission (IEC) standard defines Class -1 lasers as lasers that are safe under reasonably foreseeable conditions of operation, including the use of optical instruments for intrabeam viewing. In order to meet the Class-1 classification, no eye damage will occur even if someone looked at the laser for an extensive period of time with a magnifier in front of the laser. The maximum optical power output of a Class-1 laser inside an optical pointing device is limited by the IEC standard based on the wavelength of the laser output and the mode of operation of the laser. For example, a single mode vertical cavity surface emitting laser (VCSEL) having a nominal wavelength of 840 nanometers (nm) is defined by the IEC standard to have a peak optical power output less than 700 microwatts (μW) in a continuous wave (CW) mode to meet the Class-1 classification.
A coherent light source in a optical pointing device needs to provide a sufficient level of light output (i.e., minimum optical power output) to achieve proper exposure of the sensor image. An example minimum optical power output of a typical VCSEL in an optical pointing device is approximately 200 μW. This minimum optical power output can change with different environmental operating conditions of the optical point device. For example, less light is reflected off surfaces with darker tone and surfaces having larger surface roughness. In order to properly navigate the optical pointing device on these less reflective surfaces, the minimum optical power output from the coherent light source is typically increased. Alternatively, the exposure time of the sensor could be increased to achieve the proper level of exposure of the sensor image. The longer exposure time, however, limits the tracking speed of the optical pointing device.
Thus, the operating window of a coherent light source in an optical pointing device is defined by the minimum optical power output to provide proper exposure of the sensor image and the maximum optical power output of the coherent light source to meet the eye safety specification definition, such as the IEC standard defined Class-1 classification.
In optical pointing devices, coherent light sources, such as lasers, are typically controlled with a current regulating circuit. The current regulating circuit adjusts the optical power output of the coherent light source by varying the drive current provided by a light source driver to the light source. A typical coherent light source (e.g., VCSEL) employed in an optical pointing device is typically extremely sensitive to the drive current, such that a small fluctuation in the drive current provided by the light source driver results in a large change in the optical power output of the coherent light source. A stable and precise current source is preferably provided to the coherent light source in an optical pointing device to accommodate the operating window of the coherent light source.
One form of optical pointing device having a laser light source (e.g., VCSEL) employs a current regulating circuit to control the light source drive to provide a fixed drive current to the laser. In an optical pointing device with such fixed drive current circuitry, the mode of operation and the optical power output of the laser are based on the threshold current and the slope efficiency of the laser. The threshold current of a laser is the minimum drive current which causes the laser to start lazing. The slope efficiency of a laser is the optical power output of the laser versus drive current. VCSELs and other lasers typically employed in optical pointing devices typically have large manufacturing process variations which result in large variations in threshold current and slope efficiency of the lasers. Individual calibration of optical pointing devices is typically used for optical pointing devices with fixed current drive circuitry to ensure that the laser provides eye safe operation and minimal optical power output. Even after individual calibration, the optical power output of the laser can be affected by other parameters, such as laser age and changes in operating temperature conditions.
One form of optical pointing device which overcomes some of the above problems with fixed drive current circuitry includes closed-loop laser drive circuitry. In this form of optical pointing device, a monitoring photo diode is typically employed to continuously monitor the optical power output of the laser and provide feedback to the closed-loop laser drive circuitry. The closed-loop laser drive circuitry can accommodate threshold current and slope efficiency variations in lasers due to manufacturing process variations. In addition, laser age and operating temperature conditions can also be accommodated by the closed-loop laser drive circuitry. Closed-loop laser drive circuitry, however, is difficult and costly to implement. For example, the closed-loop laser drive circuitry employs a costly optical feedback path from the laser to the monitoring photo diode.
One form of optical pointing device includes open-loop laser drive circuitry. In one example process for manufacturing an optical pointing device having open-loop laser drive circuitry, the lasers (e.g., VCSELs) are pre-tested to determine the laser threshold current, slope efficiency, and temperature coefficient. The pre-tested lasers are sorted and grouped accordingly into a finite number of bins. Each bin of lasers is matched to a corresponding open-loop current regulating circuit. The corresponding open-loop current regulating circuit can properly adjust the drive current to the corresponding laser to ensure that the laser operates in its defined operating window to provide minimum optical power output and ensure eye safe operation. While this manufacturing process reliably ensures that the proper operating window of the laser is achieved, the manufacturing process is time intensive and costly. In addition, this manufacturing process typically results in a large percentage of the lasers being non-usable due to the limited compensation range provide by the limited number of selectable open-loop current regulating circuits.
For these and other reasons, there is a need for the present invention.